Various methods for treating of metal-based surfaces, particularly aluminium surfaces, are known in the art.
WO 03/040437 A1 relates to a composition for post-treating anodized aluminium and aluminium alloys to improve the corrosion resistance, abrasion, and adhesion bonding properties of the anodized aluminium and its alloys. The composition comprises an acidic aqueous solution having a pH ranging from about 2.5 to 4.5 containing effective amounts of trivalent chromium salts (sulphate salt), alkali metal hexafluorozirconates, an alkali metal fluoro-compound e.g. fluoroborates and/or fluorosilicates, and effective amounts of water soluble thickeners and surfactants.
WO 2011/106304 A1 discloses a process for passivating a vapor-deposited aluminium layer on a substrate, including providing a substrate comprising vapor deposited aluminium on a surface thereof; treating the surface of the substrate with an aqueous substantially chromium-free composition comprising a hexafluorozirconate.
U.S. Pat. No. 5,391,239 A discloses conversion coatings which can be formed on aluminium substrates by contacting the substrates with an aqueous liquid composition containing adequate free fluoride ion activity and concentrations of dissolved sodium and potassium, if a potassium content in the coating is desired as is usual. The aqueous liquids preferably also contain phosphate and polyhydroxy organic compounds and may contain surfactants if a cleaning effect is desired.
When treating a metal-based surface, particularly a ferrous metals surface, with one of the aforementioned methods, the corrosion resistance is not sufficient. Particularly, when the surface is treated with the aforementioned solutions prior to application of an e-coat, the appearance and durability of the e-coat is unsatisfactory.